Electrodeposition of platinum containing minor amounts of bismuth



Aug. 16, 1966 L. GREENSPAN 3,267,009

ELECTRODEPOSITION OF PLATINUM CONTAINING MINOR AMOUNTS OF BISMUTH FiledOct. 8, 1962 s Sheets-Sheet 1 FIGJ ('|:-1 Elmo-Iva smoA IVLLNI-LLOdINVENTOR. LAWRENCE GREENSPAN ATTORNEY Aug. 16. 1966 Filed Oct. 3, 1962PIC-3.2

L. GREENSPAN ELECTRODEPOSITION 0F PLATINUM CONTAINING MINOR AMOUNTS 0FBISMUTH 5 Sheets-Sheet 2 IOOO I00 g| OVERVOLTAGE 25C (c.o.AMP's./sQ.FT.)

I00 MICROINCHES) Pt-BlON Ti 50 MICRO-INCHES) P E F(P FRO 6-SAMPLE cPt-BiON Ti 7SAMPLE 0( INVENTOR LAWRENCE GREENSPAN ATTOR NEY UnitedStates Patent 3,267,009 ELECTRODEPOSITION OF PLATINUM CON- TAINING MINORAMOUNTS 0F BISMUTH Lawrence Greenspan, New York, N.Y., assignor toEngelhard Industries, Inc., Newark, N.J., a corporation of DelawareFiled 0ct..8, 1962, Ser. No. 228,935 8 Claims. (Cl. 204-43) Thisinvention relates to the electrolytic deposition of platinum fromaqueous solutions, and is particularly concerned with the preparation ofplatinum-coated base metal articles which find particular utility asnon-sacrificial 'anodes in electrochemical processes.

In electrochemical processes, the proper selection of material which maybe employed in the fabrication of electrodes is very important. In theseprocesses, it is conventional to use electrodes of iron, steel and thelike in the cathode position, but there are only a few materials whichmay be used in the fabrication of insoluble anodes, because mostmaterials, when made anodic, are susceptible to rapid corrosion. Metalsof the platinum group have desirable characteristics when utilized asinsoluble anodes, but because of the high cost of these metals, it isdesirable to use substitute materials which are less costly, andgraphite has often been employed heretofore as an anode material.

Graphite has a number of disadvantages, however, in that it undergoescontinual disintegration and must be replaced frequently, therebycausing interruption of the electrochemical process. The selection of asuitable anode is particularly critical in highly corrosiveelectrochemical baths, such as the brine solutions used in themanufacture of chlorine, and the use of graphite anodes requires theproduct to be purified for removal of traces of carbon dioxide whichresult from oxidation of the graphite anodes. Additionally,disintegration of the graphite anode causes deposition of fine grains ofgraphite in the diaphragms which surround the electrodes necessitatingreplacement of the diaphragms along with the electrodes.

The difficulties encountered with graphite electrodes, particularly inbrine electrolysis, are eliminated when platinum metal anodes areemployed. Such anodes can be of pure platinum, or, as is known in theart, can comprise a corrosion-resistant core metal such as titanium,silver, and the like, clad with a coherent, impervious platinum metalsheath or covering.

In addition to the requirement of substantial non-corrosivity, anodeswhich are employed in electrolytic processes are required to havecertain favorable current-carrying characteristics for economicoperation. Thus, the anode should be highly conductive and capable ofcarrying a high current load, i.e. operable at the highest possiblecurrent density, without undue polarization. In practice, polarizationof the anode with resultant high overvoltage characteristics, reducesthe efficiency of the electrolysis and adversely affects the economicsof the electrolytic process.

The overvoltage characteristics for chlorine discharge using platinummetal anodes, whether fabricated from sheets of precious metal orcomprised of platinum-coated base metals such as titanium, are morefavorable than when graphite anodes are used. It has been found,however, that at high current densities, egg. in excess of 200 amperesper square foot and up to about 1000 amperes per square foot, neitherplatinum sheet nor conventional platinum-coated base metals are entirelysatisfactory with respect to chlorine overvoltage characteristics.

Patented August 16, 1966 In the course of my research work directed todevelopment of a method for plating base metals with platinum whichwould provide a suitable platinum-coated non-sacrificial anode,particularly for brine electrolysis, I have noted that electrodepositsof platinum from conventional plating baths, such as hexahydroxyplatinate baths, are smooth and highly adherent. Such deposits, however,have poor overvoltage characteristics, roughly comparable to those ofplatinum sheet. I have further examined the overvoltage characteristicsof platinum black deposits, i.e., platinum which has been deposited froma bath containing minor amounts of soluble lead compounds, and havefound that such platinum black deposits provide excellent overvoltagecharacteristics. Unfortunately, platinum black deposits have thedisadvantage of extremely poor adherence, and are unsuitable forfabrication of corrosion-resistant anodes since even ordinary wipingremoves the highly corrosive-resistant coating of platinum, and exposesthe base metal.

From an examination of the coatings formed in the foregoing manner, itappeared that the excellent overvoltage characteristics of platinumblack, as compared to ordinary electrodeposited platinum, might in someway be related to the much greater active platinum surface of the morematte and relatively rough platinum black deposit. would be gained byproviding a platinum deposit of high adherence but having the matteappearance and unevenness of a platinum black deposit.

The present invention has for its principal object the provision of astable bath from which platinum deposits can be obtained which have highadherence and are characterised by favorable overvoltage characteristicsfor chlorine discharge.

A further object is the provision of a platinum-coated base metalsuitable for use as a non-sacrificial anode.

Another object is the provision of a method for depositing platinum froman electroplating bath to provide platinum-coated base metals suitableas anodes in electrochemical processes.

In accordance with this invention, adherent platinum deposits havingdesirable overvoltage characteristics are provided by depositingplatinum from an aqueous solution of chloroplatinic acid containing anexcess of hydrochloric acid and a small amount of a soluable bismuthcompound. Suitable solutions for the practice of the invention containfrom about 0.2 to about 2.5 weight percent bismuth based on a platinumin the solution. Typical solutions contain from about 5 to about 15 gm./liter platinum (e.g. as H 'PtCl -6H O), from about 0.02 to about 0.08.gm./ liter of bismuth (e.g. as BiCl and from about 1 to about 5 weightpercent hydrochloric acid. The amount of hydrochloric acid employed isnot critical, but should be at least sufiicient to provide a homogeneoussolution without formation of deposit-s during the plating process. Thebismuth may be introduced as the chloride BiCl or in the form of thehydroxide, oxide, acetate or the like, sufficient HCl being present tomaintain the bismuth in solution as the chloride.

I have found that in the absence of bismuth, firm platinum deposits canbe obtained from solution but that these deposits do not have desirableovervoltage characteristics in electrolysis of brine solutions. I havefurther found that the concentration of bismuth employed is critical,too little providing poor overvoltage characteristics, and too muchcausing formation of non-adherent, spongy deposits which are unsuitablefor the fabrication of platinum plated anodes.

These results suggested that certain advantages- Q Q? Thecharacteristics of the bismuth-containing solutions of my invention areunique, and can not be duplicated even by substitution of such closelyrelated material as antimony in the above formulation. Thus, I havetried antimony additions to solutions of chloroplatinic acid. Additionof 0.2 gm./ liter of antimony chloride (SbC-l to an aqueous solution ofchloroplatinic acid containing HCl initially resulted in a mattegrey-black deposit which was adherent. However, the effect apepared tobe transient, since subsequent samples plated from the same bath,although dark-grey in color, were smooth. Increasing additions of SbClup to 1 gram per liter, although yielding, after the addition, mattedeposits, after further electrolyzing resulted in smooth deposits havingpoor overvoltage characteristics.

Contrariwise, the bismuth-containing solutions of the invention gavestrongly adherent, uniform matte darkgrey deposits which were highlyadherent and exhibited excellent overvoltage characterisics. While thedesirable concentration of platinum and bismuth in the plating baths ofthe invention is indicated above, it will be recognized that the actualdeposits which are obtained with such solutions do not necessarily havethe same platinumbismuth ratio as present in solution. Thus, at acurrent density of 15 a.s.f. (amiperes per square foot), using asolution containing 0.06 gm./liter BiCl and 8 .gm./liter platinum as HPtOl '6H O, the bismuth content of the deposit was found to be about0.1%. In general, the bismuth content of an effective deposit will bebetween about 0.01 and about 0.1% based on platinum.

The baths of the present invention are operated at a temperature betweenabout 20 C. and about 75 C., preferably between about 20 and 60 C., andat a current density in the range of about 10 to about 30 amperes persquare foot. In the plating process, it is desirable to provide mildagitation, e.g., by gently moving the cathode, and plating is effectedfor sufficient time to provide a deposit of from about 25 to about 250,preferably about 50-100 microinches. The exact time required forobtaining deposits of the desired thickness will vary depending upon theconcentration of the solution and the current density employed, forexample, from about minutes to one hour or more.

I have noted that during the plating operation chlorine is liberated atthe platinum anodes so that, in order to employ the solution of theinvention on a production basis, adequate ventilation would have to beprovided. On continued electrolysis of the solution, the deposit changesfrom a matte dark-grey to a smoother, greyer appearance. Since the mattefinish is preferred, I have found it advantageous to bubble air or otherinert gas, e.g. N argon, etc. continuously through the solution toreduce the free chlorine content thereof, and have thus obtained moreconsistent results from the standpoint of appearance of the deposit.

To prepare metals for plating in this solution, any standard acceptedprocess may be employed, such as de greasing, electroclean-ing or otheroperations required to prepare a clean receptive metal surface.

Among the metals which may have platinum electrodeposited thereon, inaccordance with the present invention are silver, nickel, gold,tantalum, tungsten, molybdenum, titanium and rhodium. Platinum-coatedtitanium is, of course, of particular value in electrolysis of brinebecause of those characteristics of titanium including light weight,strength, corrosion-resistance and conductivity which make it of valueas a base metal for platinized anodes in brine electrolysis. For certainapplications, it may be desirable to employ the baths of this inventionto deposit platinum on a platinum or palladium base.

In order to determine the overvoltage characteristics of platinizedanodes prepared in accordance With this invention, apparatus wasemployed consisting basically of a 4: Luggin capillary probe, calomelelectrode and a vacuum tube voltmeter having a 20 megohm resistance. Theaccuracy of the measurement with this instrument is about 10-20millivolts.

OVERVOLTAGE MEASUREMENTS The Lugg-in probe consists of a tube one end ofwhich is bent at right angles and is drawn out into a fine capillarywhich makes light contact with the surface of the electrode. The otherend of the tube is connected with an open top cylindrical separatoryfunnel in which is immersed a reference electrode (calomel electrode).The funnel and tube are filled with a solution of the electrolyte, inthis case a 22% brine solution. This simple device effectively extendsthe reference electrode electrolyte (in which no current flows) up tothe surface of the electrode being studied. The leads from a vacuum tubevoltmeter are connected to this electrode and the reference electrode.With this arrangement the electrode potential is measured withpractically negligible solution IR dro T be electrolyte used was a 22%solution of sodium chloride contained in a Lucite cell. A porous claycup contained a nickel cathode and prevented mixing of solutions inanolyte and catholyte compartments. Samples for testing were prepared byplating strips of silver and titanium, 3" X 1 wide, with a deposit ofmicroinches of platinum from various solutions. The samples were maskedwith tape so as to expose a circular area approximately A3 square inchto the action of the current.

The current density range used was from 20 to 1000 amperes per squarefoot. The sodium chloride electrolyte was employed at temperatures of 25C. and 70 C.

The invention will be further illustrated by reference to the followingspecific examples:

Example I A solution was made up of the following composition:

Gm./ liter Platinum (as H PtCI -H O) 8 Bismuth (as BiCl .05 Hydrochloricacid 34 A titanium strip (3" x 1"), etched for 40 hours in concentratedhydrochloric acid, was used as the cathode. At a current density of 15amperes per sq. ft., room temperature, and providing mild agitation bymoving the cathode, the weight of deposit obtained in 15 minutes was0.23 gram, representing an average plate thickness of 100 microinches.The deposit was uniform matte, dark grey, and strongly adherent. Whenwiped with filter paper, none of the deposit rubbed off. Chlorineovervoltage measurements made in brine solution (22% NaCl) showed nopolarization occurring up to current densities of 1000 amperes persquare foot. A titanium strip similarly prepared but plated with a 100microinch deposit in a hexahydroxy platinate bath showed polarizationoccurring at 200 amperes per square foot accompanied by rapid increasein overvoltage.

Example II A solution was made up of the following composition:

Gm./ liter Platinum (as H PtCl -H O) 8 Bismuth (as BiCl .08 Hydrochloricacid 34 A strip of titanium metal (3" x 1"), etched for 48 hours inconcentrated hydrochloric acid, was used as the cathode. Using a currentdensity of 15 amperes per sq. ft., room temperature, and providing mildagitation by moving the cathode, the weight of deposit in 15 minutes was0.225 gram, representing an average thickness of 100 microinches. Thedeposit was a uniform black, but

COMPARATIVE TESTS Reference is here made to FIGURES 1, 2 and 3, attachedhereto and forming a part hereof, wherein overvoltage measurements forvarious platinized anodes are plotted as a function of current density(amps/ft?) versus potential (volts). The curves plotted in the figureswere obtained by testing the following sample structures used as anodein the test heretofore described:

Sample A.Platinum black on silver: The conventional solution forproducing platinum black was used, containing 30 gm./literchloroplatinic acid, 0.2 gm./liter lead acetate, 83 ml./ liter con. HCl(normal with respect to HCl). A current density of 15 a.s.f. was used;time of plating, 15 minutes; deposited platinum equivalent to 100microinches deposit.

Sample E.Platinum-bismuth on silver: The solution contained 20 gm./liter chlorplatinic acid, 83 ml./ liter HCl, and 0.06 gm./l'iter Bi(OH)(dissolved in HCl). A current density of 15 a.s.f. was used; time ofplating, 15 minutes; deposited thickness, 100 microinches.

Sample C.-Platinum-bismuth on titaniumas described in Example I;deposited thickness, 100 microinches.

Sample D.Platinum-bismuth on titanium-as described in Example I; depositthickness, 50 microinches.

Sample E.Platinum on titanium for platinate solution: An etched titaniumstrip was plated with platinum from a solution containing 20 gm./litersodium hexahydroxy platinate and gm./liter sodium hydroxide at atemperature of 75 C. and current density of 7.5 a.s.f.; time of plating,25 minutes; deposit thickness, 100 microinches.

Sample F.Platinum on titanium from chloroplatinic acid: An etchedtitanium strip was plated with platinum from a solution of 20 gm./literchloroplatinic acid and 83 mL/liter hydrochloric acid. A current densityof a.s.f. was used; time of plating, 15 minutes; deposit thickness, 100microinches.

Sample G.Pure platinum strip.

Sample H .Platinum deposited on silver from platinate solution: Apolished strip of silver was plated in the bath described under Sample Eat a current density of 7.5 a.s.f. for 25 minutes; thickness of deposit,10 0 microinches.

DISCUSSION Referring to FIGURE 1, wherein the overvoltagecharacteristics of certain of the sample anodes prepared as describedabove are shown as a plot of potential increase versus current density(determined at 25 C.) it will be seen from curve 1 that platinum black(Sample A) shows extremely low overvoltage values throughout the rangeof current densities tested, i.e. up to about 5-00 a.s.f. However, asstated heretofore, platinum black is non-adherent and thereforeinadequate for fabrication of corrosion resistant anodes.

Curves 3, 4 and 5 of FIGURE 1 show that platinum electrodeposited ontitanium (Sample E) or on silver (Sample H) and pure platinum sheet(Sample G) all show an undesirable rise in potential at applied currentdensity in the range of 100 to 200 a.s.f.

In marked contrast, the overvoltage characteristics of Sample B,prepared in accordance with the invention, closely approach those ofplatinum black, as shown by curve 2 of FIGURE 1, and are far superior tothose of any of the other anodes tested.

FIGURE 2 shows the results obtained by testing anodes prepared inaccordance with the invention on a titanium base in comparison with ananode prepared by depositing platinum from chloroplatinic acid in theabsence of any added bismuth compound. It will be noted that thebismuth-containing deposits (curves 6 and 7) show substantially constantpotential for current density as high as 1000 a.s.f., and that varyingthe thickness of the platinum deposit from 50 to microinches has littleeffect on the anode characteristics. As shown by curve 8, the anodeprepared by depositing platinum on titanium from chloroplatinic acid inthe absence of added bismuth showed a rapid increase in potential at acurrent density of 400 a.s.f., and would thus be unsuitable for brineelectrolysis at high current density.

Referring to FIGURE 3, comparative tests are shown for overvoltagemeasurements determined at 70 C. It will be noted that Sample C preparedin accordance with the invention showed substantially no potentialincrease up to a current density of 1000 a.s.f. (curve 9). While anodesof pure platinum (Sample G) and of platinum deposited from sodiumplatinate solution (Sample E) showed improved overvoltagecharacteristics when tested at 70 C. (curves 10 and 11 as compared tocurves 3 and 4 of FIGURE 1), neither of these samples exhibited theoutstanding flat response of the anode of this invention.

While the method of the invention has been particularly described inconnection with preparation and testing of platinum-coated anodes in theelectrolysis of brine solutions used for production of chlorine andcaustic soda, it will be apparent to those skilled in the art that theanodes of the invention can be used in diverse electroplating processes.Thus, platinized titanium anodes can be employed in electroplatingprocesses using solutions of rhodium, acid gold, nickel and the like.Other major uses for the novel anode of the invention are in theconversion of sea Water and brackish water by electrolysis, removable ofvaluable or harmful particles from industrial process water, etc.

It should be understood that this invention is not restricted to the useof any particular plating solution, apparatus or shape or form ofplatinum bismuth-coated anodes, and that the illustrative anddescriptive matter herein is presented for purposes of examplifying theinvention and, thus, should not be construed as limitative in nature.

What is claimed is:

1. A plating bath consisting essentially of an aqueous solution ofchloroplatinic acid, a soluble bismuth compound and hydrochloric acid,the bismuth being present in an amount from about 0.2 to about 2.5% byweight based on platinum.

2. A plating bath according to claim 1 containing from about 5 to about15 gm./liter platinum as chloroplatinic acid.

3. A plating bath consisting essentially of an aqueous solutioncontaining from about 5 to about 15 gm./liter platinum aschloroplat-inic acid, from about 1 to about 5% by weight hydrochloricacid, and from about 0.02 to about 0.08 gm./liter of BiCl 4. A methodfor electrolytic deposition of adherent deposits of platinum containingminor effective amounts of bismuth which comprises immersing a metal tobe plated, as the cathode of an electrical circuit into an electrolyteconsisting essentially of an aqueous solution of chloroplatinic acid, asoluble bismuth compound and hydrochloric acid, the bismuth beingpresent in an amount from about 0.2 to about 2.5 by weight based onplatinum in said solution, and passing current through the circuit.

5. The method of claim 4 wherein the deposition is effected for a periodof from hour to about 1 hour to obtain a coating between about 25 andabout 250 microinches in thickness.

-6. The method of claim 4 wherein the metal to be plated is titanium.

7. The method of claim 4 wherein the metal to be plated is silver.

References Cited by the Examiner UNITED STATES PATENTS 8/1918 Crombie204--47 X 5/1957 Atkinson 20447 8/ 1958 Dijksterhuis et a1. 204-47 X 8Miller et a1. 204-290 Newell et a1 204-290 Duva et a1. 20447 Ruff 20447X Messner 204-290 JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner.

4. A METHOD FOR ELECTROLYTE DEPOSITION OF ADHERENT DEPOSITS OF PLATINUMCONTAINING MINOR EFFECTIVE AMOUNTS OF BISMUTH WHICH COMPRISES IMMERSINGA METAL TO BE PLATED, AS THE CATHODE OF AN ELECTRICAL CIRCUIT INTO ANELECTROLYTE CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OFCHLOROPLATINIC ACID, A SOLUBLE BISMUTH COMPOUND AND HYDROCHLORIC ACID,THE BISMUTH BEING PRESENT IN AN AMOUNT FROM ABOUT 0.2 TO ABOUT 2.5% BYWEIGHT BASED ON PLATINUM IN SAID SOLUTION, AND PASSING CURRENT THROUGHTHE CIRCUIT.